<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script type="text/javascript">
        var i;
        //alert(i);//undefined
        i = 23;
        //typeof()是JavaScript语言提供的一个函数
        //它可以取变量的数据类型返回
        alert(typeof(i));//number

        i = "abc";
        alert(i);//abc
        alert(typeof(i));//string

        var a = 12;
        var b = "abc";
        alert(a * b);//NaN：非数字，非数值
    </script>
</head>
<body>

</body>
</html>